Mexico's biggest television network sold prominent politicians favourable coverage in its flagship news and entertainment shows and used the same programmes to smear a popular leftwing leader, documents seen by the Guardian appear to show.

The documents – which consist of dozens of computer files – emerge just weeks ahead of presidential elections on 1 July, and coincide with the appearance of an energetic protest movement accusing the Televisa network of manipulating its coverage to favour the leading candidate, Enrique Peña Nieto.

The documents, which appear to have been created several years ago, include:

• An outline of fees apparently charged for raising Peña Nieto's national profile when he was governor of the state of Mexico.

• Payment arrangements suggesting that the office of former president Vicente Fox concealed exorbitant public spending on media promotion.

While it has not been possible to confirm the authenticity of the documents – which were passed to the Guardian by a source who worked with Televisa – extensive cross checks have shown that the names, dates and situations mentioned largely line up with events.

There is also evidence that actions suggested in the proposals did take place. The allegations come at a crucial time for Peña Nieto, the candidate of the ideologically nebulous Institutional Revolutionary party: recent opinion polls show his substantial lead beginning to erode as Televisa's role as political kingmaker has become a central issue of the campaign.

In a country where newspaper readership is tiny and the reach of the internet and cable TV is still largely limited to the middle classes, Televisa – and its rival TV Azteca – exert a powerful influence over national politics.

Televisa, the largest media empire in the Spanish-speaking world, controls around two-thirds of programming on Mexico's free television channels. The documents appear to have been developed within Radar Servicios Especializados, a marketing company run by a Televisa vice-president, Alejandro Quintero.

Contacted by the Guardian, Televisa declined to clarify the relationship between Radar and the core company, or Quintero's role at the two companies without first seeing the documents. A spokesman refused to comment on the allegations without seeing the files. "We cannot give an opinion about imformation and/or documents we do not know," he said.

Many of the computer files seen by the Guardian were saved under the name of Yessica de Lamadrid, who at the time was a Radar employee and Peña Nieto's lover.

De Lamadrid told the Guardian that she believed the documents were forgeries. She said the promotional projects she worked on for politicians never put content up for sale.

One of the documents is a PowerPoint presentation which explicitly states its aim of making sure "López Obrador does not win the 2006 elections". That bitterly contested election saw the leftwing candidate lose a commanding lead and ended with him claiming he had been cheated.

It was apparently created just after midnight on 4 April 2005, hours before President Fox was reported to have met the heads of Televisa and TV Azteca.

Fox was facing growing criticism for an attempt to get Lopez Obrador, then mayor of Mexico City, impeached over a minor planning dispute. The document outlines short-term measures for controlling the backlash, a period of national mourning for the recently-deceased Pope John Paul II to distract attention from the growing row. The next day Fox declared a day of mourning for the pontiff.

Longer-term strategies proposed to "dismantle the public perception that Lopez Obrador is a martyr/saviour," by boosting news coverage of crime in the capital and revisiting old corruption cases involving his former allies. The plan also envisaged "promoting personal stories of crimes suffered [in the capital] by showbiz celebrities" and "urging the inhabitants of the Big Brother house" to do the same. Some Televisa celebrities did just that, both on showbiz programmes and in the Mexican version of Celebrity Big Brother broadcast that May.

The document also advises that scriptwriters of a popular political weekly satire show called El Privilegio de Mandar should make the character who represented López Obrador appear "clumsy" and "inept." The final episode of the show, broadcast immediately after the 2006 elections – when the result of a recount was still pending – ended with a non-humorous speech by an actor calling on López Obrador to accept defeat.

A former Televisa employee, who is not the source of the documents, told the Guardian that they attended meetings within the company where the anti-López Obrador strategy was discussed. "There was a strategy and there was a client who paid a lot of money," the source said.

Most of the other documents are strategies and associated budgets apparently aimed at promoting political clients through TV adverts and programmes.

They include three Excel spreadsheets titled "Enrique Peña Nieto: Budget 2005-2006" apparently created at the start of his term as governor of the state of Mexico.

All three spreadsheets detail nearly 200 news reports, interviews and features. The earliest version puts the total cost of these services at 346,326,750 pesos (about $36m at the time, or £23m today). The latest includes a "50% rate reduction".

A paper document containing the same figures seen by the Guardian was cited by López Obrador during the a presidential debate last month, in which he repeated claims that Peña Nieto was a TV product. Peña Nieto and Televisa suggested the document – first published in the left-leaning news magazine Proceso in 2005 – could have been a forgery.

The document was obtained by investigative journalist Jenaro Villamil who has always refused to reveal the identity of his source. In the past Televisa has accused Villamil of being on a mission to smear the company.

Asked if the state of Mexico had ever paid for coverage on Televisa, Peña Nieto's campaign team refused to comment. In a written response, David López, who is Peña Nieto's head of communication and previously held the same post at the state of Mexico, said: "During Enrique Peña Nieto's term as governor of the state of Mexico (from 2005 to 2011) no contract existed of that kind." Lopez added that "all the publicity contracts for the communication of government activities and the sums involved have been transparent and put on the internet."

Mexican politicians have long been criticised for spending lavishly to promote the achievements of their administrations, amid suspicion of creative accounting that masks the real cost to the public.

Media expert Raúl Trejo said the kinds of practices detailed in the document did not appear to be illegal under Mexican law, but, if true, would be unethical. The only document detailing services apparently delivered refers to a TV campaign ahead of President Fox's fifth state of the nation address on 1 September, 2005.

The document describes an "agreed fee" of 60m pesos (around $6m) covering the production of six TV adverts featuring Fox, as well as media training for five of his ministers and a series of interviews with them. The Guardian has verified that at least three of the interviews took place. The section which could prove most controversial refers to arrangements for payment, which suggest deliberate manipulation to conceal the extent of spending. The document says that "as agreed" the presidential office has been billed directly for only 3m pesos, adding that invoices for the remaining 57m pesos will be sent out when "the presidential office tells us which other parts of the government to bill for what services."

The files also contain proposals, budgets and promotional material involving several other politicians including the former minister of Tamaulipas state, Tomás Yarrington accused by US prosecutors of laundering money for the Gulf drugs cartel. Yarrington's lawyers have denied the allegations of money-laundering.

Another politician mentioned in the documents, former senator Demetrio Sodi, said he had no knowledge of a promotional strategy drawn up by Radar shortly before his unsuccessful bid to become mayor of Mexico City.

Sodi said it was unlikely that the document was a forgery, but insisted that he had never paid for favourable coverage. He suggested the document might have originated among people who, unknown to him, wanted to support his candidacy.

None of the other politicians named in the documents would talk to the Guardian. The current wave of protests against perceived media dirty tricks was triggered on 10 May when Televisa first ignored an anti-Peña Nieto protest at a private university where he was giving a campaign speech – and then gave wide coverage to accusations that the protest was staged by non-student troublemakers.

One protester at a recent demonstration carried a placard proclaiming: "Not even my mother manipulates me like Televisa."

As the protests against alleged media bias gather pace, Televisa has become keen to prove that its coverage is balanced. It now covers the protest movement in detail, and the anchors of its main news shows recently put Peña Nieto through a gruelling interview. The network has also announced that it will broadcast the next presidential debate on 10 June on its most popular channel, which during the first debate was reserved for a talent show.

The former Televisa employee said that while the network was happy to promote Peña Nieto when "he was the best product," this did not necessarily mean long term commitment. The source pointed out that prior to the dirty tricks campaign against him, Lopez Obrador was on very good terms with the network."Never lose sight of the fact that this is a business. The loyalty is to the position, not to the person."